It has been known for almost 200 years that muscle contraction can be controlled by applying an electrical stimulus to the associated nerves. Practical long-term application of this knowledge, however, was not possible until the relatively recent development of totally implantable miniature electronic circuits which avoid the risk of infection at the sites of percutaneous connecting wires. A well-known example of this modern technology is the artificial cardiac pacemaker which has been successfully implanted in many patients.
Modern circuitry enables wireless control of implanted devices by wireless telemetry communication between external and internal circuits. That is, external controls can be used to command implanted nerve stimulators to regain muscle control in injured limbs, to control bladder and sphincter function, to alleviate pain and hypertension, and to restore proper function to many other portions of an impaired or injured nerve-muscle system.
To provide an electrical connection to the peripheral nerve which controls the muscles of interest, an electrode (and sometimes an array of multiple electrodes) is secured to and around the nerve bundle. A wire or cable from the electrode is in turn connected to the implanted package of circuitry. The present invention is directed to an improvement in this type of electrode.
A widely used prior-art electrode assembly is formed from a tube of silicone rubber with one or more electrodes secured on the inner surface of the tube. An end-to-end slit is cut through the tube sidewall so the tube can be opened and fitted over the nerve bundle. When so installed, the resiliency of the tube causes it to surround the nerve bundle to urge the electrode against the surface of the tissue, and the tube may also be provided with suture flaps for additional anchorage. Due to its construction, this style of assembly is usually called a "cuff" electrode.
Animal-implant studies suggest that cuff electrodes can cause neural damage, and are not wholly satisfactory for long-term implantation. The probable causes of these problems can be summarized as follows:
A. Although having some radial flexibility to enable installation over the nerve, the silicone-rubber tube or sleeve is relatively stiff to insure that the restoring force of the resilient material will position the electrode against the nerve surface to insure adequate electrical contact. Excessive gripping and compression of the nerve by the cuff can cause nerve damage by decreasing blood and axoplasmic flow, and by constricting nerve fibers with resulting loss of function. This problem is accentuated by temporary swelling of the nerve caused by the trauma of surgical implantation of the electrode. PA1 B. If a cuff electrode is loosely fitted to limit pressure atrophy of the nerve, a poor electrical contact is made, and this contact is further degraded in time by ingrowth of connective tissue between the cuff and nerve. This ingrowth is sometimes sufficiently marked to lead to compression damage to the nerve as discussed above, or it may cause complete separation of the cuff and nerve. PA1 C. The nerve is encased within the full length of the cuff, blocking a normal metabolic exchange between the nerve and surrounding tissue. That is, a normal and desired fluid interchange between the nerve and its surrounding environment is prevented or sharply decreased over the length of the cuff. PA1 D. In addition to compression damage, mechanical trauma to the nerve can be caused by torque or bending forces applied by the cuff and its relatively stiff cable during muscle and body movement. These forces may even displace the nerve bundle out of the cuff. PA1 E. Conventional cuff assemblies use electrodes of small surface area, and the resulting high density of electrical charge at the electrode-nerve interface can result in an undesired electrochemical deposition of electrode material on the nerve sheath.
The helical electrode of this invention overcomes or minimizes the problems which have been observed with implanted cuff electrodes. Broadly, the new electrode assembly includes a spiral or helical array configured to fit around a nerve bundle, and having at least one plate, foil, or wire electrode on its surface. A flexible cable connects the helical array to an implanted biostimulator electronics package.
During surgical implantation, the softness and pliability of the spiral array enables it to be gently wound around the nerve with minimal nerve manipulation and constriction of blood vessels. Any post-operative edema or swelling of the nerve is accommodated by a gentle yielding and radial expansion of the spiral which minimizes the risk of compression atrophy of the nerve tissue. The open construction of the helix significantly reduces coverage of the nerve periphery to promote a more normal fluid exchange with surrounding tissue. A good electrical contact between electrode and nerve is maintained, and the problem of growth of connective tissue into the electrode-nerve interface is minimized.
In further contrast to cuff electrodes, the spiral array completely encircles the nerve to insure contact with sub-bundles within the main nerve bundle. The resiliency of the array and connecting cable provide good isolation of the nerve from mechanical loads during body and muscle movement. Importantly, the effective surface area of each electrode is made larger than its plane geometric area by a peening or other roughening operation to reduce electrical charge density at the nerve-electrode interface.